Sound translating apparatus



June 28, 1938. s. BALLANTINE 2,121,779

SOUND T RANSLATING APPARATUS Filed Feb. 12, 1955 4 Sheets-Sheet 1 June 28, 1938. 5, BALLAN-HNE 2,121,779

' souun TRANSLATING APPARATUS Filed Feb. 12, 1935 4 Sheets-Sheet 2 I EIMMMMM June 28, 1938. w s. BALLANTINE 2,121,779

SOUND TRANSLA'I'ING APPARATUS Filed Feb. 12, 1935 4 Sheets-Sheet 3 l o/faqe e" loo who may Freqae/w/ 3 t June 28, 1938 S. BALLANTINE 2,121,779

SOUND TRANSLATING APPARATUS Filed Feb. 12, 1935 451188tS-Sh86t 4 fiequen cy Patented June 28, 1938 UNITED .STATES PATENT OFFICE 21 Claims.

This invention relates to electric communication systems, and particularly to microphones for the transmission of voice sounds.

Ordinary types of microphones, designed to be 5 actuated by aerial sound waves, must be supported in front of, and preferably close to, the mouth. There are many applications and uses for microphones, however, where this is quite in- ,convenient even in those cases where some means 10 of support, other than the hands, are provided. Examples of such use are in gas and oxygen masks. According to this invention, I abandon the method of picking up the sound from the air and instead pick up the sound as it exists in the vibrations of the upper parts of the body, which are set up by the acoustical and mechanical actions of the voice. These vibrations are particularly strong in the region of the larynx and associated cartilaginous structures such as the thyroid cartilage. The invention is characterized by a device that is so applied to the vibratory parts as to be directly actuated by the mechanical vibrations thereof, and acoustical transmission through the air plays a minor role.

" An object of the invention is to provide a device of novel construction for converting acoustical vibrations of the body due to the voice into electric currents for transmission in an electrical communication system. An object is to provide a microphone, of the character stated, which operates on the piezoelectric principle and which therefore does not require for its operation any external electrical or magnetic sources such, for example, as polarizing voltages, magnetic fields or the battery ordinarily used with microphones of the carbon granule type.

Further objects are to provide larynx microphones which are characterized by a simple and rugged mechanical construction and by an electrical output which compensates, in part at least, for the deficiency of the higher frequency constituents in the mechanical vibrations of the larynx.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings in which:

Fig. 11s a front elevation of an embodimen of the invention in place on the operators neck;

Fig. 2 is a perspective view of the double unit microphone and supporting band;

Fig. 3 is a longitudinal sectional view through the microphone, one piezoelectric unit being shown in central section;

Fig. 4 is a front view, with parts broken away, of a single unit;

Fig. 5 is a similar rear view of a unit;

Fig. 6 is a transverse section through a unit;

Fig. '7 is a fragmentary view, partly in section, of the inner side of the microphone covering;

Figs. 8 and 9 are perspective views of a Rochelle salt crystal and a plate cut therefrom, the views illustrating the method of cutting the piezoelectric elements for use in the microphone;

Figs. 10 and 11 are respectively, exploded and and assembly views, in perspective, of a crystal assembly or piezoelectric unit;

Fig. 12 Ba fragmentary diagram of a circuit for determining the resonant frequency of a crystal assembly;

Figs. 13 and 14 are, respectively, curve sheets showing the frequency response characteristics of acrystal assembly and of a microphone unit;

Fig. 15 is a transverse section through a microphone, in place upon the larynx, and illustrating a satisfactory construction of the microphone covering; A

Fig. 16 is a similar section showing an alternative construction; and

Fig. 1'7 is a. transverse section of a modified construction which includes material for damping vibration of the crystal assembly.

The new microphone is adapted to be worn. around the neck like a collar, as shown in Fig. 1, but other methods of support may be employed. In the particular embodiment of the invention which I have selected for purposes of illustration, the microphone M has the general form of a narrow band in which two mechano-electrical transducers are symmetrically arranged, and an adjustable elastic band B is sewed to the microphone to form a collar that may be placed around the neck in such a way that the microphone units are in contact with the neck in the region of the larynx. The covering of the microphone may be of leather, fabric or other suitable material, including solid material such as molded synthetic resins, for example molded resins of the types sold commercially under the trade-mark Bakelite".

Besides protecting and holding the units in place, this covering also reduces noise due to aerial sound waves and the impact of wind against the microphone; as a result and also because of the fact that the microphone is designed to be actuated by mechanical vibrations rather than aerial sound waves, the microphone is very insensitive to extraneous noise. This is an important advantage when used in noisy locations, such as open cockpit airplanes, where the noise picked up by microphones of conventional types has been a serious problem. With my microphone, used in an airplane, the motor and propeller noises are almost inaudible; the only extraneous noise heard is a low intensity sound due to the impact of wind against the microphone, not loud enough to be troublesome. The noise due to wind in airplanes may be further reduced by wearing a muflier over the microphone and this is convenient in practice since a muiiier is generally worn anyway.

The construction of the transducers and their arrangement in the microphone assembly are best shown in Figs. 3 to 'I. The covering I may be formed of suitable material, such as glove leather, with a snap fastener 2 and fiber stiifener I fastened to one end of the casing. The side of the microphone which contacts with the neck will be designated, for convenience of description, as the "front of the microphone. The front wall i is provided with two holes which are covered by a thin membranous material that may be cemented to the front wall leather, and the actuating buttons 5 of the microphone units extend through these holes. Not all materials are suitable for the membrane 4 on account of attenuation of the higher frequency vibrations. Materials which have been used successfully are thin rubber, animal intestinal linings which may be grouped under the name of colon leather", and ordinary leather of the order of .005" to .015" thickness. Thin glove leather has been employed in most of my constructions because of its good mechanical qualities and because its physical appearance can be made to match the rest of the leather covering. This is cemented to the covering I with rubber cement and extends over the entire area of the side toward the neck. In order to reduce the strain on the covering l and the thin protective covering 4, a thin metal grommet B is sewn and/or cemented around the edge of the openings in the front wall I, see Fig. '7.

The transducer units are of substantially identical construction and include a slab-shaped assembly I of Rochelle salt crystals which is supported and clamped at its ends between separators 8, 9. The selection of the material for these separators depends upon the frequency characteristic which is desired as the damping of the crystal assemblies 1 may be controlled somewhat by the choice of materials. I have found rubber, leather, and certain synthetic resins to be suitable. For example, synthetic resinous bodies of the class produced from glycerol and organic acids and designated as the Glyptal type may be employed. The mechanical vibrations of the body are communicated to the crystal assemblies by means of the buttons 5, and these also affect the frequency characteristic to some extent. Where it is desired to transmit the higher frequencies, the buttons should be rigid and preferably of some light material. Balsa wood has been found suitable. The buttons are attached to stirrups Ill made of light metal and cemented to the crystal slab, the stirrups serving to reduce The whole assembly is enclosed in a metallic housing or shell I i, and is protected and clamped in place by the metallic cover I! that is attached to the housing by screws l3. Any vibrations communicated to the cover II by wind or aerial sound waves of external origin are undesirable. This is discouraged by the protective covering l, and may be further discouraged if necessary by making the cover I! of a material having a high ratio of elastic viscosity to Youngs modulus. Hard copper is superior to aluminum for this pudpose and may be employed when trouble of this sort is experienced. An end wall ll of the nousing is a block of insulating material, such as hard rubber, a molded synthetic resin or the like, which is fastened to the metallic shell by screws. The electrical connections to the crystal slab are made by conductors it which are held in the block ll. The metallic parts of the housing serve as an electrical shield and are normally connected to ground. It may be desirable to ground the wearer of the microphone also, and this is accomplished by metallic inserts it which extend through the front wall, leather l and membrane 4, and are electrically connected to the grounded parts of the microphone. These inserts contact with the skin when the microphone is worn.

I have found that when it is desired to obtain an efficient response at the higher frequencies, as is the case with throat microphones, it is of advantage to make the metallic housing II more robust than would be required merely for purposes of mechanical support. This control of the frequency-response characteristics is apparently connected with the flexure and vibration of the housing which, by its inertia, holds the ends of the crystal relatively stationary while the middle portion of the crystal is vibrated by the button 5. Accordingly, I have made the back of the housing beneath the crystal of generous cross-section as shown in the side views of Fig. 3 and in section in Fig. 6.

Electrical connections to the two microphone units are made by means of the cable H which extends along a channel or groove in the rear walls of the housings Ii. This is preferably shielded electrically and connections are made to an inner conductor and to the metallic shield. The crystal units may be connected in series or in parallel, care being taken to see that the outputs are electrically additive. The parallel connection is somewhat more convenient. One terminal of each crystal is connected to the shield of the cable and also to the metallic housing, the other terminals each connect to the innerconductor of the cable. The cable is anchored in place by the stirrup-shaped metal piece 18 which is riveted to the housing of one of the units, thus removing the mechanical strain from the connections. The cable i1 is preferably fastened in the channel in the housing I l in some way. The method shown in Figs. 3 and 5 is suitable, con sisting merely of cementing a piece of fabric 19 over the back of the housing.

In order to diminish the chance of breakage of the crystal assembly 1, due to accidental applications of excessive pressures on the buttons, a

stop I, Fig. 6, is provided which limits the bendvibration intensities are seldom the same on the two sides of the larynx and this effect varies in different individuals, probably on account of anatomical variations in structure. In such circumstances, the use of two units averages the response.

In the second place, the vibration intensities will usually vary with different locations. Here again the averaging eiiect obtained by the use of two units makes the position of the microphone on the neck much less critical than would be the ease with a single unit.

The structure and operation or the crystal assembly i may be briefly described as follows: Fig. 8 shows a homogeneous crystal of Rochelle salt from which a slab is cut, as shown by the dotted lines, with its faces normal to the electric, or c, axis. The piezoelectric activity of such a slab is illustrated in Fig. 9. When an electric field is set up along the c axis, the plate is subjected to shearing stress which causes an extensional strain in a direction at 45 to the b and c axes and at the same time a contractional strain in a direction 90 from this. Two slabs are cut, as shown by the dotted lines in Fig. 9, with their long sides parallel to these strains and after being provided with foil electrodes are cemented together as'described by Sawyer in U. 8. Patent No. 1,802,782. An exploded view of the assembly is shown in Fig. 10, and an isometric view of the complete assembly in Fig. 11. Thin metallic foil electrodes E are attached to the outer surfaces of the crystal slabs S and these slabs are then cemented together with the foil E interposed to form the assembly shown in Fig. 11. Electrical connections are made to the projections t, t of the foils, the two outer foils being connected together, thus placing the units in parallel. The slabs are so connected and poled that when an electrical voltage is applied between t and 1.", one slab tends to elongate and the other to contract. This causes the whole assembly to bend. Conversely, if the assembly is bent an electrical voltage is generated. This action is depended upon for the operation of the microphone. No claim is made forthe novelty of this crystal assembly, which is described in the Sawyer patent above cited.

The crystal assembly is protected from the effects of moisture by coating it with a special varnish. After some experimenting, a successful material for this purpose has been discovered, consisting of No. Bakelite varnish.

The outer electrodes E of the assembly (terminal t) are connected to ground and the middle electrode E (terminal t) to the inner conductor of the cable; This improves the electrical shielding and reduces capacitative coupling due to flux leakage through the'hole in the cover l2, Fig. 4.

The relation between the voltage generated by the microphone and frequency of the vibration imparted to buttons is afiected by the dimensions of the crystal. As outlined in my copending application Serial No. 6,245, filed concurrently herewith, I have made an experimental study of the amplitudes of the vibration of the larynx at different frequencies and have discovered that the higher frequencies are very weak. The high frequency components are necessary for the transmission of intelligible speech sounds because the consonant sounds are largely composed of such components. Accordingly, in microphones foruse on the throat, I restore this balance between the high' and low frequency components by designing the microphone to have as much response at the higher frequencies as possible. This is achieved in the present case by dimensioning and clamping the crystals so that they have a natural frequency of vibra- 5 tion in the upper part of the frequency range which it isdesired totransmit. Slabs of the following dimensions are suitable: length 1.25 inches, width 0.5 inch and total thickness 0.12

inch. in the microphone, slabs of these dimenslons have resonances in the neighborhood of 4000-8000 cycles depending upon the rigidity of the housing.

It will be understood, however, that these dimensions are given only by way of example as the same resonant frequencies may be obtained by other combinations of length, breadth and thickness, as is well known from the theory of elastic vibrations in slabs. It is also to be remembered that the resonant frequencies depend to some extent upon other factors-the size and weight of the buttons, mechanical impedance of the throat, clamping of the slab and form of the metallic housing. Theoretically, the resonant frequency of an elastic slab of this type is proportional to t/i where t is the thickness and l the length. This law may be used as a rough guide in design. When measured in centimeters, the dimensions given in the preceding paragraph would make the ratio approximately equal to 0.3.

Values between 0.15 and 0.6 have been found suitable, depending upon the clamping, buttons and housing employed.

The resonant frequencies may be determined by the method indicated by Fig. 12. Here e represents a source of electromotlve force of variable frequency, 0 represents the crystal slab mounted in the microphone and R a resistor of about 500 ohms resistance. If the voltage e is maintained constant and the output voltage e' is plotted against frequency, a curve of the type shown in Fig. 13 is obtained. The resonant vibration of the crystal causes a'serration in the curve at A due to the back electromotive force generated by the vibration of the crystal (motional impedance). In taking the data for this curve, the microphone was worn on the neck in order that the normal mechanical impedances would be applied to the buttons.-

The frequency response characteristic of a.

typical microphone of the described construction is shown by curve B, Fig. 14. Data for this curve was obtained by measuring the open-circuit generated voltage of the microphone, for

different frequencies, when the button 5 was in contact with a piston that was driven sinusoidally at constant velocity for all frequencies. The term constant velocity means that the maximum value of the velocity, which.v varies sinusoidally, is maintained constant at all frequencies.

If sufficiently high response at the higher audio frequencies cannot be obtained in the microphone alone, a further increase in response at these frequencies may be obtained in an associated electrical network, as described and claimed in my copending' application.

The efficacy of this throat microphone may be judged from the fact that consonant articulations of 95 percent have been obtained using standard word lists.

This articulation may be compared with that obtained in standard commercial telephone systems where articulations of 70-75 percent are usual.

Noise due to the entrance of air into thespace I construction. In making up the casing, a roll strip 20 is first sewed to the edge of the front strip by stitching II, the front and back strips I are placed upon each other and the strip 20 is rolled back upon itself to enclose the meeting edges, and the assembly is completed by stitching 22 which passes through the rear edge of strip 20 and the two strips I of the casing. These rolled strips ,20 rest against the neck, indicated at 23, and

form a seal against the wind.

A good seal may also be formed, as shown in Fig. 16, by sewing or cementing a pad 24 to the forward edge of the leather casing I. Sponge rubber or a soft felt, such as corn felt, are suitable materials for the pad. A layer 25 of sound insulating material may be positioned between the housing I I and the rear wall I to reduce noise due to the impact of the wind directly upon the microphone. Various materials, for example "corn" felt, have been used successfully in this position to reduce noise. I

Constructions of these types for eliminating or reducing the effects of wind and air pressure may be used in throat microphones having transducer elements that differ materially from those herein described. Claims to throat microphones having means to prevent wind and aerial sound waves from reaching the transducer element or elements are presented in my co-pending application Ser. No. 166,667, filed Sept. 30, 1937, Throat microphones.

As has already been mentioned, some damping of the vibrations of the crystal assembly may be obtained by means of the materials used for the clamping strips 8, 9. An alternative method of obtaining a desired damping is to insert pieces of damping material 26, 21, 28, between the crystal assembly I and the housing II, as shown in Fig. 17. Any material exhibiting elastic viscosity or hysteresis may be employed, such as felt, sponge rubber, or felt soaked in oil, petroleum jelly or other viscous fluids.

While I have described a preferred embodiment of the invention, it will be apparent that there is considerable latitude in the design and construction of the device and that variations, additional to those herein specifically mentioned, may be made without departure from the spirit of my invention as set forth in the following claims.

I claim:

1. In a sound translating device adapted to be actuated by vibrations of the body due to the voice, a mechano-electrical transducer, a substantially rigid member for mechanically transmitting the vibrations of the body to said transducer, and supporting inertial means enclosing said transducer and member, said supporting means including a flexible membrane of low acoustical attenuation extending over said member.

2. In a sound translating device, a pair of mechano-electrical transducers, an inertia-providing casing housing each of said transducers, members contacting the respective transducers and extending to the exterior of the casings thereof, and means for symmetrically supporting said transducers on the throat and with said members positioned to transmit vibrations of the throat to prising a flexible band enclosing said casings and adapted to encircle the neck, the portions of said band overlying the vibration-transmitting members being thin membranes of low acoustical attenuation.

3. In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, terminals thereon for connecting to an electric circuit, means for supporting the ends of the crystal assembly, and mechanical means for contacting the central portion of the crystal assembly to transmit thereto the vibrations of the body.

4. In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, a housing substantially enclosing the said crystal assembly and supporting the ends thereof, and motion-transmitting means adapted to contact with the body, said means extending into said housing to engage the central portion of said assembly to impart a vibratory bending motion thereto when said means is vibrated by the body.

5. In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, a rigid support extending lengthwise of and parallel to said crystal assembly, means for clamping said assembly to the opposite ends of said support, and means contacting the central portion of the crystal assembly and adapted to contact with the body for mechanically transmitting the vibrations thereof to said crystal assembly, thereby to impart a vibratory bending motion to said crystal assembly.

6. In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, a rigid support, means for clamping the two ends of said crystal assembly to said support, and means adapted to contact with the body for transmitting the vibrations thereof to the said crystal assembly approximately midway between the two clamped ends, whereby a vibratory bending motion is imparted to the said crystal assembly.

7. A microphone, adapted to be actuated by vibrations of the body, comprising a slab-shaped piezoelectric crystal assembly sensitive to bending, a rigid support, means for clamping said crystal assembly at its ends to said support, and a solid piece fastened to one of the faces of said crystal assembly midway from the ends thereof and adapted to be placed in contact with the body, whereby a vibrating bending motion Is imparted to the crystal assembly by the vibrations of the body.

8. In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, a rigid metallic housing tenclosing said crystal assembly, means clamping the two ends of the assembly to said housing, and a button protruding through a hole in said housing and fastened to one of the faces of said crystal assembly midway from the ends thereof, said button being adapted to be placed in contact with the body, whereby a vibrating bending motion is imparted to the crystal by the vibrations of the body.

9..In a microphone adapted to be actuated by vibrations of the body, a slab-shaped piezoelectric crystal assembly sensitive to bending, a housing substantially enclosing said crystal assembly, means contacting said crystal assembly at end portions of at least one side thereof. for supporting the same, and means engaging spaced poxtions 0! said crystal assembly for damping the vibration thereof, and motion-transmitting means contacting said crystal assembly at the central portion thereof and extending to the exterior of said housing for contact with the body.

10. In a microphone, a slab-shaped piezoclectric crystal assembly sensitive to bending, a

and said crystal assembly for damping the vibrations thereof. I

13. A microphone comprising two piezoelectric crystal assemblies. each assembly comprising a pair of superposed crystal slabs contacting with a central electrode and having outer electrodes at their outer faces, a metallic housing enclosing each of said assemblies, a shielded cable for connecting the microphone to an external circuit, said cable comprising an insulated central conductor and a concentric shield, electrical connections from theiinner electrodes of each crystal assembly to the central conductor of the said cable, electrical connections from the outer electrodes of each crystal assembly to the shield of said cable, and electrical connections between the shield 0! said cable and each of said metallic housings.

14. In a microphone adapted to beactuated by vibrations of the body due to the voice, a piezoelectric crystal assembly sensitive to bending, means supporting the ends of said crystal assembly, and means secured to the center of said assembly for mechanically transmitting the vibrations oi the body thereto, the said crystal assembly and means having. a principal resonant frequency at the higher audio frequencies.

15. In a microphoneadapted to be actuated by vibrations of the ,body due to the voice, a piezo electric crystal assembly, a rigid support having an inertia substantially greater than that of said crystal assembly, means for clamping the ends of said crystal'assembly to said support, and means for mechanically transmitting vibrations oi the body to the said crystal assembly, the said crystal assembly and vibration transmitting means being of small mass,and having a principal resonant frequency at the higher audio frequencies.

16. In a microphone adapted to be actuated by vibrations of the body due to the voice, a piezoelectric crystal assembly sensitive to bending, a rigid support, means for clamping the ends of said crystal assembly to said support,- and means for mechanically-transmitting vibrations of the body to the said crystal assembly, the said crystal assembly having a principal resonant frequency above 1500 cycles.

11. m a microphone adapted to be actuated by vibrations of the body due to the voice, a plum electric crystal assembly, a rigid support having crystal assembly, means for clamping both ends of said crystal assembly to said support, and means for mechanically transmitting vibrations of the body to the said crystal assembly, the said crystal assembly and vibration transmitting means being of small mass and having a principal resonant frequency in the neighborhood of 5000 cycles. Q l

18. In a microphone adapted to be'actuated by vibrations of the body due to the voice, a slabshaped Rochelle salt piezoelectric crystal assembly sensitive to bending, a rigid support, meansfor clamping both ends of said crystal assembly to said support, and means. for mechanically transmitting vibrations of the body to the said an inertia substantially greater than that of said crystal assembly, the dimensions of said crystal assembly being of the order of 1% inches long and 5; inch thick.

18. In a throat microphone adapted to be actuated by vibrations of the body due to voice, a transducer comprising means of relatively low inertia adapted to be vibrated and means of relatively higher inertia for supporting said vibrating means, means contacting said vibrating means for transmitting thereto vibrations of the body due to the voice, and means for supporting said transducer on the body, said vibrating means and vibration-transmitting means having a principal for transmitting thereto 'vi brationsot the body due to thevoice, and means for supporting said transducer on the body, said vibrating means and vibration-transmitting means having a principal resonance frequency at the upper end of the range of audio frequencies.

21. A microphone comprising a piezoelectric crystal assembly, said assembly comprising a pair of superposed crystal slabs contacting with a central electrode and having outer electrodes at their outer faces, a metallic housing incompletely enclosing said assembly, a shielded cable for connecting the microphone to an external circuit, said cable comprising an insulated central conductor and a conductive shield, electrical connections from the inner electrode of the crystal assembly to the central conductor 01 thesaid cable, electrical connections from the outer electrodes of the crystal assembly to the shield or said cable, and an electrical connection between the shield of said cable and the metallic housing.

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